1. Field of the Invention
The present invention relates generally to telecommunication circuits. More specifically, the present invention relates to full-duplex speakerphone control circuits including a supplementary echo suppressor.
2. Description of the Related Art
Most modern speakerphones use half-duplex operation which switches transmission between the far-end talker and the near-end speakerphone user. System designers resort to half-duplex operation because the acoustic coupling between the speaker and microphone is much higher in speakerphones than in a handset where the coupling is mechanically suppressed .
Hands-free communication through a microphone and speaker typically results in acoustic feedback or howling because the loop gain of the system exceeds unity when audio amplitudes are adjusted to a reasonable level.
Howling is a condition occurring in full-duplex operation in which both the microphone and speaker are active at the same time so that, in conjunction with the reflection off the hybrid, a closed loop is created. The signal coupling between the speaker and the microphone causes feedback oscillation or howling when the coupling between the speaker and microphone is strong enough to increase the system closed loop gain above unity.
The solution to the howling problem has typically been half-duplex operation, in which either the transmit channel or the receive channel is active with both channels never active at the same time. Half-duplex operation prevents howling but diminishes the overall communication quality by clipping words and forcing the speaker at each end to wait for the speaker at the other end to stop talking.
In full-duplex conversation, both transmit and receive channels are active simultaneously. Telephone handsets allow full-duplex conversation quality.
Another technique for addressing acoustic howling that results from the loop becoming unstable due to a high loop-gain is to utilize an "echo suppressor" that operates in combination with an echo canceller to substantially reduce residual echo. The echo suppressor includes a first attenuator in the transmit path and a second attenuator in the receive path. If either attenuator is activated, the loop-gain is reduced to prevent acoustic howling. Control logic for the attenuators is typically designed so that at least one attenuator is on at one particular time. In echo conditions, the attenuator in the path from the near-end microphone to the far-end is designed to have at least enough attenuation to prevent echo that is objectionable to the far-end user. The control logic is designed so that the attenuator is continuously active when potentially objectionable echo is present.
Conventional systems including echo cancellers in combination with echo suppressors typically do not adequately accommodate full-duplex communication. Echo cancellers and suppressors that do support full-duplex communication generally have difficulty determining which signals are speech signals and which are echo. These echo suppressors therefore have difficulty in determining which signals to suppress.